Automotive air conditioning compressors of the swash plate type reciprocate a plurality of cylindrical pistons back and forth within close fitting cylinder bores. The edge of the nutating swash plate slides through, and rocks within, a ball-shoe joint formed in the body of the piston. The swash plate, being disposed at an angle to the shaft, produces a somewhat unbalanced force on the piston, a force that tends to tilt the pistons off axis, especially at full retraction. The ball-shoe joint, since it has radial clearance built in to it, is not capable of preventing the pistons from cocking radially inwardly and away from the axes of the cylinder bores as they retract. There are also side loads acting on a piston that can tend to drive it off axis in the lateral direction, perpendicular to the radial direction. In many compressors, the pistons are double sided, with two opposed ends sliding back and forth simultaneously in opposed, collinear cylinder bores. An example may be seen in co-assigned U.S. Pat. No. 4,351,227. With double ended pistons, the piston body as a whole is kept securely on axis by the fact that its two widely spaced ends ride back and forth in the opposed bores, like a beam supported at both ends.
It is also possible to drive single ended pistons with a swash plate, a design suitable for smaller capacity, more axially compact compressors. With only the front end supported in the bore, and the back end relatively unsupported, the piston is more subject to tilting off axis as it retracts, especially radially inwardly. One proposed solution is the addition of an extra swash plate, running in tandem to the first, with rod and ball joints extending from the back ends of the pistons to the tandem plate to help guide and align the pistons as they retract. This is a very expensive proposal, and would require a longer compressor housing. An extra swash plate and rod joints would also add a good deal of extra weight.
Two other potential solutions to the problem of piston axis misalignment, disclosed in co pending, co-assigned U.S. patent applications, provide new piston designs which have inherently better axial alignment. U.S. Ser. No. 08/210,489, filed Mar. 21, 1994, and allowed Dec. 6, 1994, adds stability with a piston that is non circular in cross section, as is its matching cylinder bore. This design is primarily directed toward radial compressor compactness, however. U.S. Ser. No. 08/338,298, filed Nov. 14, 1994, discloses a piston which, while still cylindrical, has an effectively longer front end. Therefore, when fully retracted on the backstroke, there is more piston front end surface area still remaining in contact with the cylinder bore, and thus more resistance to the piston cocking off axis. Both designs require significant changes to the standard piston shape and structure, especially to the piston front end or head. A way to provide better piston alignment without significant change to the shape or length of the piston, and without increasing compressor housing length, would be desirable. Such a design could be easily retro fitted to existing compressors.